This description relates to imaging and to devices in lithographic systems, methods, and techniques.
Lithography is a process in which features are rendered on generally flat surfaces. The features can be rendered using lithography masks (also known as “reticles”) that modulate the exposure of a workpiece surface by electromagnetic radiation. The image projected on the workpiece by mask-modulated radiation is stored as a latent image in photosensitive media. After additional processing, the latent image is transformed into features on the workpiece that are used in further processing. For example, such features can be used to form integrated circuits on semiconductor wafers.
Phase shifting systems, methods, and techniques used in lithography vary the phase of electromagnetic radiation propagating through the mask to improve image quality of the features patterned on the workpiece. These improvements can include increased spatial resolution of printed features, increased contrast, and improvements resulting from an increased depth of focus during printing.
Phase shifting is commonly implemented using phase shifting masks. Phase shifting masks act to change the phase of transmitted radiation and cause the interference effects that improve feature quality at the workpiece. Phase shifting masks generally include two or more regions that a given frequency of electromagnetic radiation traverses in a differing average number of wavelengths. When the radiation traverses such regions, the differing number of optical wavelengths changes the phase relationship of the transmitted radiation. Using appropriate mask layout techniques, these changes in phase relationship can yield the improvements described above.
Two categories of phase shifting masks include alternating phase shifting masks and embedded phase shifting masks. Alternating phase shifting masks generally include two or more associated apertures with different transmissive properties. In particular, radiation from a first of the aperture(s) generally arrives at a given point at a workpiece with a phase differential relative to radiation from a second aperture. The apertures can be associated so that the phase differential results in interference effects that improve the quality of the printing.
Alternating phase shifting masks can include a phase shifter with a refractive index that is higher or lower than the bulk of the mask. The phase shifter can increase or decrease the average number of optical wavelengths along an optical path through a selected mask aperture to modulate the phase relative to other transmitted radiation. A phase shifter in a traditional alternating mask causes a minimal or no change to the intensity of such radiation relative to the intensity of radiation passing along other optical paths. Traditionally, alternating 180° phase shifting masks are produced by etching portions of the bulk of a mask substrate to a predetermined depth. Mask substrates are commonly selected to be highly transmissive to a chosen frequency of electromagnetic radiation.
FIG. 1 shows a theoretical radiation intensity distribution 100 of an image formed using an alternating phase shifting masks. Radiation intensity distribution 100 can be obtained using a phase shifting grating with equal width opaque lines and transparent spaces. Every other transparent space transmits electromagnetic radiation 180° out of phase from the light transmitted through the other transparent spaces. Radiation intensity distribution 100 includes points 105, 110 which are midway between adjacent spaces and where the radiation intensity is near zero where the electric field of the transmitted radiation changes sign.
Embedded phase shifting masks generally include regions that reduce the relative intensity of transmitted radiation while simultaneously changing the phase relationship of the reduced intensity radiation relative to other transmitted radiation. Such regions can include, e.g., thin films including chrome, CrxOy, CrxOyNz, MoxSiyOz, or MoxSiyOzNq. Embedded phase shifting masks are also known as attenuated phase shifting masks or transmission-π masks.
Other example categories of masks include alternating aperture phase shifting masks (PSM), hidden shifter PSM, exposed shifter PSM, chromeless PSM (phase-edge PSM), chromeless shifter-shutter, high transmission embedded PSM, leaky chrome PSM, and rim shifter masks.
Like reference symbols in the various drawings indicate like elements.